A Research On Mechanical Properties For Braided Reinforcements Of Composites During Three-dimensional Preforming Process

The composites with complex shapes have been gained more attention and applications since the pursuit of materials in the balance of performance,reliability and light-weight is being chased by advanced manufacture industries such as aviation,automobile and defence.The braided fabrics,which are produced by textile technique,have been deemed as the promising reinforcements to replace woven ones in manufacturing composites with complex shapes,owing to fast fibre deposition and well versatility.But,the mechanical theory in manufacturing the braided composites with complex shapes by far is insufficiency,such as the mechanical and deformability behaviours before and after preforming process,resulting in a restriction on further applying the braided composites with complex shapes.This is also an urgent-solved problem in manufacturing composites with desired performance and high efficiency at present.Therefore,this thesis based on theoretical and experimental analysis tackles such problem above.The first part of this thesis consists of Chapter ? and ? focusing on the mechanical properties of braided reinforcements.The second part of this thesis including Chapter IV and V mainly concentrates deformability behaviours during the preforming process.In Chapter II,the most important mechanical behaviour of braided reinforcements,in-plane shearing,was completely analysed conducted by theoretical modelling and experimental verification based on bias-extension test.It was suggested from comparison between theoretical and experimental data that such a model could well describe the in-plane shearing of braided reinforcements.Besides,the in-plane shearing is a unique behaviour and independent of fabrics size.However.the in-plane strength showed asymmetry in transversal and axial directions of fabric,which is dominated by the braiding angle of braided fabrics.Hence,Chapter III utilized braiding angle and fibre number of axial yarns as variables to figure out the mechanical behaviours of triaxial braided reinforcements by sort of bias-extension test,revealing the relation of basic parameters of reinforcements and mechanical behaviours.Firstly,the strength of triaxial braided fabrics presented "two peaks",which were correlated with axial yarns and in-plane shearing of bias yarns,respectively.Moreover,the strength was positively proportional to fibre number of axial yarns,negatively proportional to the braiding angle.That was also shown in temperature of fabrics during the tests.Besides,the temperature greatly raised when the bias yarns compressed each other during in-plane shearing.The first part of this thesis conducts a fundamental role in exploring the deformability behaviours of braided reinforcements during following preforming process.In Chapter ? and ?,the deformability behaviours of braided reinforcements during the preforming process were defined and analysed.In Chapter ?,the deformability behaviours and defects were qualitatively analysed using the preforming tests with different preforming pressures.As one of the most important deformability behaviours,the yarns sliding can be defined in two yarn directions,longitudinal and radial directions.The gap and bulking,which are defects usually detected during preforming,were induced by excessive yarns sliding along radial yarn direction.The excessive yarns sliding along longitudinal yarn direction resulted in one special defect,fibre vacancies.Besides,axial yarns and bias yarns showed the difference in sliding,and sliding was not impacted by preforming pressure.Out of the preforming zone,the in-plane shearing zone can be detected,and the extent of in-plane shearing deduced as increasing preforming pressure.In Chapter V,the mechanical models to drive the deformability behaviours were proposed based on the qualitative analysis in Chapter 4.It was verified that braiding angle has a great effect on deformability behaviours.Moreover,the law behind the variation of deformability behaviours induced by changing braiding angle was also explored.Being aimed at describing each yarn sliding,the geometrical model considering the punch tool shape and basic parameters of reinforcement was proposed and verified using the exqperimental results.It was suggested that the geometrical model can be partly character the bias yarns sliding,but well describe the axial yarns sliding.The method to explore the rule behind the variation of in-plane shearing was also put forward.This study not only justifies the feasibility that braided reinforcements could be used to composites with complex shapes manufacturing,but also put forward an innovative method to machining composites with the high quality and efficiency.